1. Field of the Invention
The present invention relates to a power supply circuit that divides power from a commercial power supply and supplies the power to loads, which is favorable for use as a power supply for an electric device, for example, an LED device, such as an indoor/outdoor lighting device, an electronic billboard, colored lights or a signboard, or a heater device such as a floor heater or a snow-melting system.
2. Description of the Related Art
In recent years, devices using LEDs, which have long-life and low-power consumption, have commonly been distributed for the aforementioned electric devices. For such a power supply circuit for an LED device, for example, the switching power supply disclosed in Japanese Patent Laid-Open No. 2007-274751 is known.
In the switching power supply in Japanese Patent Laid-Open No. 2007-274751, a main circuit thereof includes: a noise filter circuit including capacitors and coils; a rectifier/smoother circuit including diodes and capacitors; a radiator-equipped power switching element such as a field-effect transistor, and a radio-frequency transformer. Also, in addition to the main circuit, the switching power supply includes a control circuit, an overvoltage protection circuit and an overcurrent protection circuit, and the electronic components included in these circuits are mounted on a circuit board, which is housed in a metal case.
In the aforementioned switching power supply, an input voltage from a commercial power supply is stepped down by means of the radio-frequency transformer to adapt the voltage to a rated voltage for LEDs. However, in the radio-frequency transformer, an iron core is heated by an eddy current generated in the iron core, causing an energy loss called an iron loss. In particular, such iron loss is large in a radio-frequency transformer. Furthermore, the radio-frequency transformer also causes another energy loss, which is a copper loss caused by the resistances of conductors. These iron and copper losses result in a problem in deterioration in energy efficiency of power supply.
Also, in the aforementioned switching power supply, the radiator-equipped power switching element is employed for dissipation of heat in the radio-frequency transformer, and the overvoltage protection circuit and the overcurrent protection circuit are provided for prevention of an LED failure. However, since a large number of electronic components are required for providing these elements and circuits, a circuit board with a large size is needed, causing a problem in causing an increase in size of the power supply. Furthermore, for a method for suppressing the aforementioned energy losses, the current density of the conductors in the radio-frequency transformer may be decreased; however, in this case, it is necessary to increase the wire diameter of the conductors, increasing the weight of the radio-frequency transformer, as well as providing another factor in causing an increase in size of the power supply.
The present invention has been made in view of such problems as described above, and an object of the present invention is to provide a power supply circuit enabling energy efficiency enhancement and downsizing of a power supply.